Silenes chemical reactions

Summary Using a new synthetic pathway, four intramolecularly donor-stabilized silenes, R(Me3Si)Si=C(SiMe3)2 (10 R = 8-dimethylamino-l-naphthyl 11 R = 2 -(dimethylaminomethyl)phenyl 14 R = 2,6-bis(dimethylaminomethyl)phenyl 15 R = 2,6-bis(diethylaminomethyl)phenyl)), were prepared by the reaction of (dichloromethyl)tris(trimethylsilyl)silane (Ic) with the respective dialkylamino substituted aryllithium compounds (molar ratio 1 2). X-ray structural analyses of the four silenes revealed strong donor-acceptor interactions between the dialkylamino groups and the electrophilic silene silicon atoms, leading to pyramidalization at the silicon centers. The configuration at the silene carbon atoms was found to be planar. The chemical behavior of the new silenes, particularly reactions with water and methanol, treatment with methyl iodide or methyl triflate and conversions with benzaldehyde, are discussed. 

This review describes the current status of silenes (silaethylenes, silaethenes), molecules which contain a silicon-carbon double bond. The heart of the material is derived from a computer-based search of the literature which we believe reports all silenes that have been described to date, either as isolated species, chemically trapped species, proposed intermediates (in reactions where some experimental evidence has been provided), or as the result of molecular orbital calculations. Ionized species... 

The facile photochemical sigmatropic 1,3-trimethylsilyl shift in polysilylacylsilanes from silicon to oxygen (equation 33) was utilized historically to prepare the first relatively stable silenes3 86 87. Silenes prepared by isomerization of acylpolysilanes bear, due to the synthetic approach, a trimethylsiloxy group at the sp2-hybridized carbon and relatively stable silenes of this type have in addition also at least one trimethylsilyl group at the silicon. These substituents strongly influence the physical properties and the chemical behaviour of these silenes. This is noticeable in many reactions in which these Brook -type silenes behave differently from simple silenes or silenes of the Wiberg type. 

Encounters between silyl radicals in solution or in the gas phase usually result in recombination and disproportionation (45, 46). Disproportionation results in the production of silanes and highly reactive silenes. The disproportionation reaction is thermodynamically favorable because of the formation of a silicon-carbon double bond, which, although subsequently chemically reactive, is worth —39 kcal/mol (44). For pentamethyldisilanyl radicals, disproportionation is kinetically competitive with radical dimerization (46). In an earlier study, Boudjouk and co-workers (47) demonstrated conclusively by isotopic substitution and trapping that the silyl radicals generated by photolysis undergo disproportionation, as well as, presumably, dimerization (Scheme I). In deuterated methanol, the silanes produced were predominantly undeuterated, whereas methoxymethyldiphenylsilane was extensively deuterated in the a position. The results of these experiments strongly implicated the substituted silene produced by disproportionation. 

---